The present invention relates to constant-current sources and, in particular, to a transistor constant-current source having an applied voltage reference that compensates for temperature variations in the junction conduction voltage of the transistor to provide a constant output current independent of temperature.
Integrated circuits extensively employ balanced differential amplifiers, which require the use of a controlled constant-current source. Temperature-compensating networks are necessary in the design of a constant-current source to ensure that the gain, DC operating point, and other important characteristics of the amplifier will vary as required over the operating temperature range. These characteristics are also sensitive to variations in the bias voltage applied to the amplifier.
Differential amplifiers used in integrated logic circuits typically employ a transistor that functions as a constant-current source. In the case of a bipolar transistor, a voltage applied between its base and emitter terminals produces a flow of electrical current through its collector terminal. In the absence of compensation of some type, the collector current can change with variations in the bias voltage applied to the transistor or with temperature changes in the base-emitter diode junction of the transistor. These variations can adversely affect the performance of the integrated logic circuits by causing changes in the peak-to-peak output voltage excursions and, as a consequence, changes in the operating characteristics, such as noise margin and propagation delay. Such changes in operating characteristics are unacceptable in circuits that employ many logic circuits which operate in synchronism to accomplish a predictable logic function. Applying a regulated reference voltage to the base-emitter diode junction of the transistor will not prevent such changes in operating characteristics from occurring.